1. Technical Field
The present invention relates to a polymerizable ink composition, an ink cartridge containing the polymerizable ink composition, and an inkjet printer.
2. Background Art
In the field of inkjet printing, the field of industrial inkjet printing in particular, curable inkjet ink has been diffusing in terms of quick-drying, wide variety of printing substrates (recording media), and being free from volatile organic compounds (VOC).
Curable inkjet ink contains unsaturated organic compounds polymerized by, for example, exposure to electron beams or ultraviolet ray under the presence of a photopolymerization initiator. Such curable inkjet ink produces radicals upon irradiation of energy and the unsaturated organic compounds come to have high molecular weights by the radical reaction. As a result, the ink is fixed by curing or solidification. This reaction instantly proceeds upon irradiation of energy, which results in quick drying.
In addition, since curable inkjet ink is solidified immediately, the ink can be printed on a non-ink absorbing print substrate (recording medium) that does not absorb ink such as metal, glass, china and porcelain, ceramics, or plastic film without degradation of the quality of image, resulting in a wide selection of print substrates.
Furthermore, since the entire of the unsaturated organic compounds become large polymers, the printing process using this ink is VOC-free.
As the unsaturated organic compound (hereinafter referred to as monomer) of curable inkjet ink, (meth)acrylic acid ester compounds are used in general. Most of such (meth)acrylic acid ester compounds are viscous. Typically, ink using a multi-functional (meth)acrylic acid ester compound having a number of polymerizable functional groups has a high cross-linking density and forms images hard to remove. However, the ink tends to be highly viscous, which makes it difficult to overcome a trade-off between lowering the viscosity of a curable inkjet ink and improving the curing property thereof.
In attempts to solve this problem, JP-2010-509426-A discloses a curable inkjet composition that can prevent curing inhibition caused by oxygen and improve attachability by adding a monomer having an epoxy group cured by a cation, a cationic photopolymezation initiator, and water.
JP-2008-507598-A discloses ink that can form high definition images at high speed because the curing speed thereof is improved by setting each content of an unsaturated ethylene mono-functional monomer, an unsaturated ethylene di-valent monomer, an unsaturated ethylene multi-valent monomer, and a urethane ethylene oligomer in particular ranges.
JP-2009-272609-A discloses a curable composition that contains a compound having a methacroyl group or acroyl group that is able to prevent clogging of nozzles caused by curing of ink, a compound having a thermocuring functional group such as a hydroxyl group or a carboxylic group, and a dilution agent such as a vinyl ether or an ethylene derivative.
JP-2010-530922-A discloses ultraviolet ray curable ink that improves attachability to polypropylene or a non-porous substrate having a low surface tension by adding a monoacrylate having a glass transition temperature Tg to improve the flexibility of ink layers.
JP-2007-231230-A, JP-2007-231232-A, and JP-2008-68516-A disclose ink compositions that improve the attachability between a recording medium and ink by adding an ester or amide of (meth)acrylic acid, a vinyl ether having a hydrocarbon group, and a polymerization initiator.
JP-3672651-B1 (JP-H09-183927-A) discloses an ultraviolet ray curing resin composition for inkjet recording that forms images on paper or a plastic substrate without forming a resin layer for attachment by setting the mass ratio among a (meth)acrylate compound, a vinyl ether compound, and a photopolymerization initiator in a particular range.
JP-2009-62541-A, JP-2005-532445-A, and JP-2004-526820-A disclose ink having a quick curing speed and a low viscosity, which can be obtained by using α,β-unsaturated ether together with a (meth)acrylate monomer.
In general, most of (meth)acrylic acid ester compounds are toxic potential about skin sensitization that cause an allergy by touching (one of reaction of delayed hypersensitivity that causes irritated skin by excessive immune reaction to a chemical material). That is, if non-cured monomer compositions remain on a printed matter, it may cause an allergy by touching with a finger or a hand. For this reason, ink that forms safe printed matters without causing an allergy is desired.
However, as described above, in addition to the trade-off between lowering the viscosity of a curable inkjet ink and improving the curing property thereof, manufacturing ink having no or little skin sensitization problem is extremely difficult. In fact, the skin sensitization problem is not considered in the patent documents mentioned above.
In an attempt to solve this problem, JP-2012-251123-A discloses a photopolymerizable inkjet ink containing a (meth)acrylate and/or a (meth)acrylamide, and a vinyl ether compound, which has an excellent curing property and a low viscosity without skin sensitization. Although this is successful to some degree, the storage stability thereof is not found to be satisfactory.
Furthermore, 2013-040280-A discloses an addition of water or a tertiary amine compound to improve storage stability.